box: introduce system privileges

Add system privileges 'session' and 'usage'

* 'session' privilege lets user connect to database server
* 'usage' privilege lets user use his/her rights on database objects
* Both privileges are assigned to all users by default.

Implementation details:
* system privileges are special grant rights to 'universe'.
Therefore, they can be granted only by admin. Because of this fact,
during creation or deletion of user, we have to switch to 'admin' to
grant or revoke these rights.

Important changes:
* changed bootstrap.snap due to need to start admin with new privileges
* added auto upgrade script for 1.7.7

Fixes gh-2898.

With contributions by @kostja.